Troubleshooting for Recurrent Solenoid Burn Out
FLUID POWER - Design Data Sheet 18
This sheet describes an easy-to-assemble test device to help
diagnose the cause of repeated burn out of a solenoid coil on a
double solenoid air or hydraulic valve.
One possible cause for recurrent burn out is the energizing of
both solenoids at the same time. Even a short overlap of a small
fraction of a second, if repeated cycle after cycle, may eventually
build up enough heat to break down the coil insulation or burn out
This overlap of energization could be a fault in the electrical
circuit design, or it could be improper operation of an electrical
component - such as sticking or delayed break of relay
This type of burn out can only occur on double solenoid valves
of the direct-acting type - including the direct-acting piggy-back
operators on top of large pilot-operated bodies where the two
solenoids are mechanically yoked to opposite ends of the same
spool, and would "fight" each other if both were energized at the
same time. One of them would be able to seat and would be subject
only to holding current. The other would be held open, causing it
to draw inrush current, usually about 4 to 5 times normal holding
current, and it would soon burn out.
There are other possible causes for recurrent solenoid coil burn
out, and these will be discussed in a later issue.
First, wire the coils of Relays CR1 and CR2 to the valve solenoids
so the relays
will beenergized and their contacts will close each time the
valve solenoids are energized.
Figure 2. Next,
wire Relay CR1 and CR2 contacts in series with coil of CR3, so
contacts will close only if there should be current on both
valve solenoids at the same time.
How the Test Device Operates
The test device described here is very simple, consisting of three
relays and a pilot lamp. Each relay has one set of normally open
(N.O.) contacts. Two relays have their coil wired to one of the
valve solenoids, as in the left diagram. They pull in when the
valve coil to which they are wired is energized. The third relay is
connected to a signal lamp which gives the alarm if at any time,
even momentarily, both valve coils become energized at the same
time. Voltage rating of the first two relays must be the same as
the solenoid coils to which they are connected.
Each time one of the valve solenoids is energized, the relay
wired to it will close its contacts. However, Relay CR3 coil will
only be energized if both sets of contacts on Relays CR1 and CR2
close at the same time, since they are wired in series. If Relay
CR3 should become energized, its contacts will electrically lock it
closed where it cannot open even though the valve coils become
de-energized. The signal lamp also lights through contacts of CR3.
Once it lights, it will remain lighted indefinitely until the
operator unplugs the test device, allowing Relay CR3 to
Try this device next time you have a case of mysterious
recurrent burn out for which you cannot find the reason.
SELF-SHIFTING OF 4-WAY HYDRAULIC VALVES
In some fluid power circuits there could be a tendency for a
4-way valve spool to shift accidentally even though the valve
remains unactuated. We call this "self-shift". This undesirable
action could be a safety hazard or it could interfere with normal
functioning of the machine on which the valve is installed.
Cause of Self-Shift
Several factors may contribute to this type-of malfunction machine
vibration, gravity weight of a valve spool operating in a vertical
direction. But probably the primary cause is a pressure unbalance
developed within the valve body by excessive flow across the spool
grooves. It may happen infrequently, once an hour, once a day, or
once a month, with normal operation for long periods in between
malfunctions. Self-shift can be minimized or eliminated by good
Simple reciprocation circuit which may be subject to self-shift if
not properly designed.
Circuits Susceptible to Self-Shift
Figure 3. Self-shift is most likely to occur in
hydraulic circuits which use a double solenoid, 2-position,
no-spring valve in which the solenoid is momentarily energized to
shift the valve spool and thus to reverse the travel direction of
the cylinder such as in this simple reciprocating circuit.
As the cylinder rod travels forward the cam actuates the limit
switch, energizing Solenoid B, and shifting the valve to retract
the cylinder. When the cylinder cam backs off of the limit switch,
Solenoid B becomes de-energized, and the valve spool is left in an
unrestrained condition and subject to possible self-shift. If
self-shift should occur, the cylinder may stop as soon as it backs
off the limit switch because the valve spool has been moved into
its closed crossover position. The circuit can be restored by
momentarily re-actuating the limit switch.
Balanced spool areas A and B are exposed to unequal pressures while
fluid is flowing.
Pressure Unbalance Causes Self-Shift
Figure 4. In this exaggerated cross-section of a
spool valve, internal fluid pressure is working against equal and
opposite areas, "A" and "B", on the sides of the spool groove. As
long as fluid is not flowing, both areas are exposed to equal
pressure, and the spool remains in perfect pressure balance. But
when fluid starts flowing across the spool groove there will be a
pressure drop created by the flow. If there should be, for example,
a 50 PSI drop created by the flow, then when 1,000 PSI is present
at the inlet port and working on spool Area "A", there will be 950
PSI at the outlet port and working on spool Area "B". This
difference of 50 PSI multiplied by the groove sectional area may
cause the spool to drift toward the right unless it is firmly held
against drift by fluid, mechanical, or solenoid force.
With the spool and body construction used in most 4-way valves,
the pressure unbalance through one flow path (pres. to cylinder)
will be opposed by an unbalance in the opposite direction caused by
the other flow path (cylinder to tank). When using a double-end-rod
cylinder with equal areas on opposite sides of the piston, the
self-shift forces would seem to be cancelled. However, most
cylinders have unbalanced areas, so the flow through the two paths
would be different, causing a net unbalance of forces acting on the
Design Precautions for Avoiding Self-Shift
Especially during the design stage, give these factors careful
- Keep within the maximum GPM rating of the valve as published by
its manufacturer. Excessive flow through a valve not only creates
abnormally high pressure and power losses, but may cause
self-shift, and may even cause premature solenoid coil burn-out in
spring centered or spring return models. Remember when sizing a
valve that the highest flow occurs through the return passage while
the cylinder is retracting, and is up to twice the pump flow on 2:1
ratio single-end-rod cylinders.
- When using double solenoid, 2-position valves as in Figure 3,
design the electrical circuit, if possible, so current is held on
one solenoid or the other during cylinder travel.
- Mount all 4-way valves with spool horizontal.
- If using a 2-position, no-spring model, select one with detent
action against the spool.
- On pilot-operated type solenoid valves, if an external drain
line is required, do not tee this line into the main tank return
line; run the drain separately to tank.
Download a PDF of Fluid
Power Design Data Sheet 18 - Troubleshooting for Recurrent Solenoid
Coil Burn Out.
© 1990 by Womack Machine Supply Co. This
company assumes no liability for errors in data nor in safe and/or
satisfactory operation of equipment designed from this